Burner for Particulate Fuel

ABSTRACT

Disclosed is a burner for particulate fuel, in particular made of biomass, with a primary tube and a core tube arranged in the primary tube. The primary tube and the core tube form a primary tube gap and the primary tube gap is configured to guide a flow of particulate fuel and gaseous combustion means from an inlet-side end to an outlet-side opening of the primary tube. In order to prevent the drawbacks occurring when using coarse-grain particles, preferably biomass, as a fuel for dust firing, or at least to reduce them without having to accept an increased outlay for equipment and/or additional energy losses, at least one device is provided for centring the flow within the primary tube in the region of the outlet-side end of the primary tube.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The invention relates to a burner for particulate fuel, in particularmade of biomass, with a primary tube and a core tube arranged in theprimary tube, the primary tube and the core tube forming a primary tubegap and the primary tube gap being configured to guide a flow ofparticulate fuel and gaseous combustion means from an inlet-side end toan outlet-side opening of the primary tube.

2) Description of Prior Art

Burners for the combustion of particulate fuels, such as, in particular,coal, in a combustion chamber have been known for some time. Dust firingis also referred to in this connection.

A burner of this type is described, for example, in EP 0 571 704 A2. Theburner has a core tube, which has air flowing through it, and has aburner gun for igniting the particulate fuel. Arranged concentricallywith respect to the core tube is a primary tube, which, with the coretube, forms an annular gap, which is connected at its rear end to a dustline. A mixture of coal particles and primary combustion means (primaryair) is supplied via the dust line to the burner. The mixture of coalparticles and combustion means is made to rotate by means of a swirlingbody arranged in the annular gap, so the coal particles are concentratedin the outer region of the annular gap.

Additionally provided concentrically with respect to the primary tubeare a secondary tube and a tertiary tube, which, with the respectiveinner tube, define a secondary and a tertiary annular gap, which havesecondary and tertiary combustion means (secondary air and tertiary air)flowing through them. Swirling bodies are also provided in the secondaryand the tertiary annular gaps in order to impress a swirl on thecombustion means. Conical widenings in the wall of the combustionchamber are provided at the outlet-side end of the secondary tube andthe tertiary tube.

Provided at the outlet-side end of the primary tube is a so-calledflame-holder, which has a radially inwardly directed edge leading tostalling and to turbulence of the coal particles. Thus, a flow isproduced, which is directed into the combustion chamber, with a highdegree of turbulence and coal particle concentration. This flow is“surrounded” by the flows leaving the core tube, the secondary annulargap and the tertiary annular gap. Owing to the high degree of turbulenceof the particle-rich flow, the volatile components are very rapidlyexpelled from the coal particles. Because of the high particleconcentration, the air ratio is strongly sub-stoichiometric, so lessnitrogen oxides (NOx) are formed.

The burners of the type mentioned can basically also be used to burnparticulate fuels other than coal, for example biomass. For thispurpose, the biomass has to be very finely ground, however, which,because of the usually fibrous and tough structure of conventionalbiomasses, is linked with an increased outlay for equipment and energy.In particular, the fine grinding of biomass often entails a high degreeof wear of the equipment used for this. Biomasses are therefore notgenerally ground so finely as coal. In the case of hard coal, theparticle size is typically 90% smaller than 90 μm and, in the case ofbrown coal, 90% smaller than 200 μm. On the other hand, in biomass, amean particle size of about 1 mm is desired.

The volatile components of the biomass particles are already expelledmore slowly because of their size, which can impair stable combustion ofthe biomass. In addition, a correspondingly larger quantity of air, theso-called carrying air, has to be used in order to transport the largerbiomass particles, free of deposits, from the crusher through the burnerinto the combustion chamber. The larger carrying air quantity flowingthrough the annular gap between the primary tube and the core tube can,together with a delayed release of volatile components, lead to a localair excess during the combustion. As a result of this, more nitrogenoxides are formed.

The present invention is therefore based on the object of preventing thedrawbacks occurring during the use of coarse-grain particles, preferablybiomass, as a fuel for dust firing, or at least to reduce them, withoutan increased outlay for equipment and/or additional energy losses havingto be accepted.

SUMMARY OF THE INVENTION

This object is achieved in a burner of the type mentioned at the outsetand described in more detail above in that at least one device isprovided for centring the flow within the primary tube in the region ofthe outlet-side end of the primary tube.

The invention recognised that the drawback of an increased carrying airquantity and larger particle diameters in the combustion of coarse-grainfuels, such as biomass, can be in any case partially compensated by afluidic deflection of a part of the primary air in the direction of thecore zone of the burner mouth. The deflection makes it possible to guidea part of the primary air around the flame-holder or to guide itcentrally through the latter, without this part of the primary airarriving in the turbulent particle flow zone adjoining the flame-holder.This only takes place at a later point in time, at which the turbulentparticle flow zone has widened and the volatile components of the fuelparticles have escaped to a greater extent. The particle concentrationis consequently high in the particle flow downstream of theflame-holder. Consequently, the flame of the burner can be stabiliseddespite a delayed escape of volatile components. In addition, the oxygenconcentration in the particle flow downstream of the flame-holder isclearly sub-stoichiometric, which counteracts the formation of nitrogenoxides.

The deflection of a part of the primary air in a central region of theburner is made possible by the swirling of the primary air in theprimary annular gap, which concentrates the fuel particles in the outerregion of the primary annular gap and feeds them to the flame-holder.The concentration of the fuel particles in the outer region of theprimary annular gap is accompanied by a depletion of particles in theinner region of the primary air flow. The invention makes use of theinvention to divert a part of the primary air into the central region ofthe burner without this having significant effects on the transport ofthe fuel particles. However, it is necessary for the partial diversionof the primary air flow to adapt the burner geometry in such a way as toprovide space for the primary air flow to be deflected. This space isnot present in conventional burner geometries.

According to the invention, it is unnecessary for the fuel particles tobe transported by an air flow through the primary tube gap even if thisis appropriate for cost reasons. Instead of air, another combustionmeans known per se could also be used. It would even be conceivable touse an oxygen-free gas if the oxygen required for the combustion isotherwise provided. For the sake of simplicity, the term primary air isused, however, below.

There are also basically no limits with regard to the core tube. Gashaving oxygen or an oxygen-free gas can flow through the core tube,which may, in particular, be expedient to cool the core tube. As analternative or in addition, a burner gun can be received in the coretube to provide a support or ignition flame. Quite in general, coretubes may be provided which are constructed in a manner known per se. Asa result of the deflection of a part of the primary air flow, ifnecessary, a flow through the core tube can be dispensed with. This mayalso favour the deflection of the primary air into the central region ofthe burner behind the core tube.

The person skilled in the art will recognise that the core tube and theprimary tube preferably have circular cross-sections and are arrangedconcentrically with respect to one another as this is favoured in termsof flow technology. The core tube and the primary tube then form aprimary tube gap in the form of a symmetric annular gap. Basically,there could be a deviation both from circular cross-sections of the coretube and primary tube and/or from a concentric arrangement of thesetubes, even if, as a rule, this is less preferred. However, for the sakeof simplicity, in the present case, only the terms core tube and primarytube are used instead of core channel and primary channel without thisinevitably having to be interpreted in a restrictive manner.

In a first configuration of the invention, the core tube ends before theprimary tube, viewed in the longitudinal direction of the burner. Theinner part of the primary air can therefore arrive on time in front ofthe flame-holder in a central region of the burner and, uninfluenced bythe flame-holder, in a flow that is laminar as far as possible, bypassthe latter there. So an inner part of the primary air can reach thecentral region or core region of the burner and can form a uniform flowas far as possible, an adequate spacing has to be provided between theoutlet-side end of the core tube and the outlet-side end of the primarytube or the flame-holder—if present. In the longitudinal direction ofthe primary tube, this spacing should be at least 50% of the mean radialwidth of the primary tube gap. From a flow point of view, it is morefavourable, however, if this spacing is at least 75%, in particular atleast 100%. A very short spacing in comparison to this may becounter-productive, in particular if the core tube ends abruptly. Theflow close to the core tube in the primary gap can be stalled there andextend the turbulent region after the flame-holder. Thus, specificallyno part flow of the primary air flow is guided around the flame-holderand the turbulent region of particle-rich flow adjoining the latter.

Alternatively or in addition, the core tube may taper toward itsoutlet-side end. The tapering of the end of the tube, compared to anabrupt end of the core tube, has the advantage that the flow can beguided more uniformly. Stalling and turbulences can thus be avoided inthe region of the inner part of the primary air flow. It is particularlypreferred if a tapering of the core tube is accompanied by alongitudinal-side spacing of the flame-holder or opening of the primarytube, on the one hand, and the outlet-side end of the core tube, on theother hand.

For flow reasons, the core tube may taper constantly toward itsoutlet-side end. The tapering may be uniform or non-uniform here. It isfavourable in terms of flow technology if the tapering decreases to theoutlet-side end in order to avoid stalling before the end of the coretube.

To even out the flow, it is preferred, in particular in the case of around core tube, if the latter tapers conically toward its outlet-sideend. In this case, the angle of inclination of the cone should not betoo great to avoid a stall. Angles of inclination of less than 20° arepreferred here. In order to avoid a stall even at higher flow speeds,angles of inclination of less than 10° are appropriate. During testing,particularly good results were achieved with angles of inclination ofabout 7°, if necessary with a deviation of ±1°.

To support the deflection of a part of the primary air, a deflectiondevice may be provided at the outlet-side end and outside the core tubein the primary tube to deflect the flow guided close to the core tube inthe primary tube gap inwardly. As a result, it can, for example, beensured that the desired proportion of primary air is also diverted inthe direction of the centre. In addition, the deflected part flow can beguided in a more laminar manner owing to the additional surfaces of thedeflection device. The deflection device preferably projects into theprimary tube gap, in particular into the primary air flow.

In order to avoid a deflection of the fuel particles into the coreregion of the burner, the deflection device may be configured to deflectabout 30% by volume to 70% by volume of the air flow in the primary tubegap. In this case, it is appropriate if the deflection deviceapproximately extends into the primary tube gap, preferably radially, toover 30% to 70% of the width of the primary tube gap. Particularly goodresults are achieved if the deflection device is configured to deflectabout 40% by volume to 60% by volume of the air flow in the primary tubegap and/or extends therein to over 40% to 60% of the gap width of theprimary tube gap.

Provided between the core tube and the deflection device is preferably aflow channel, through which the deflected primary air flow is guided. Inthis case, it is particularly preferred from the technical flow point ofview if the free flow cross section in the flow channel of thedeflection device remains constant. An unfavourable variation withrespect to energy of the flow speed can thus be avoided.

As an alternative or in addition to further devices, at least one flowdirector may be provided in the primary tube gap to influence the swirlof a part close to the core tube of the flow guided in the primary tubegap. A widening of the primary air flow after leaving the primary tubegap can be counteracted, for example, by influencing the swirl of atleast the flow close to the core tube, which favours the centring of thepart flow of the primary air close to the core tube. A plurality of flowdirectors, preferably distributed over the periphery of the primary tubegap, may also be provided. The number of flow directors shouldpreferably increase here with the diameter of the primary tube.

The at least one flow director may be oriented in the longitudinaldirection of the primary tube. The swirl of at least one part of theprimary air flow close to the core tube is at least weakened thereby,which can have a favourable effect on the flow conditions. In order todirect the flow but not to lastingly disrupt it, the at least one flowdirector should be much wider in the longitudinal direction than in theperipheral direction of the primary tube gap.

The at least one flow director may, as an alternative thereto, also beoriented transverse to the longitudinal direction, i.e. partially in theperipheral direction, of the primary tube. In this case, the orientationof the at least one flow director may differ from an orientation in thelongitudinal direction of the primary tube in such a way that the swirlof the primary air flow is intensified by the at least one flowdirector, at least for a part of the primary air flow close to the coretube. The at least one flow director may, however, also reduce the swirlof the primary air flow in an also possible orientation pointing more inthe longitudinal direction of the primary tube. The at least one flowdirector may, however, also be oriented in the opposite direction to theswirl direction of the primary air flow. This may, for example, lead tothe fact that the swirl direction of the primary air flow is reversed atleast for a part of the primary air flow close to the core tube. Inorder to intensify the swirl of the primary air flow in regions, it maybe expedient to incline the at least one flow director by 35° to 45°relative to the longitudinal direction of the primary tube. In order toweaken the swirl of the primary air flow in regions, it may befavourable to incline the at least one flow director by less than 25°,in particular less than 15°, relative to the longitudinal direction ofthe primary tube.

Depending on the boundary conditions in terms of flow technology, eachof these orientations of the at least one flow director may entailpositive effects. Basically, by means of a swirl, which has a differentstrength or is differently directed, of parts of the primary air flow, aseparation in terms of flow technology of these parts can be achieved,as the latter have different properties in terms of flow technology. Toenable a control of the burner, the at least one flow director may bevariable with regard to its orientation, i.e. incline compared to thecore tube.

It is also conceivable for the at least one flow director to have avarying incline in the longitudinal direction of the primary tube inorder to achieve a gradual change in the swirl direction of the part ofthe primary air flow close to the core tube. Alternatively or inaddition, however, a plurality of flow directors or groups of flowdirectors distributed over the periphery of the core tube may also beprovided one after the other. In this case, it is particularly preferredif the incline changes relative to the longitudinal direction of theprimary tube from flow director to flow director or from one group offlow directors to the next group of flow directors in the longitudinaldirection of the primary tube.

Moreover, it can easily be achieved that the swirl of the outer flow inthe primary tube gap remains uninfluenced by the at least one flowdirector in order to not impair the flame stability. For this purpose,the flow director is not provided in the last outer 20% of the primarytube gap. If the outer 30% or even 40% of the primary tube gap can bekept free of flow directors, this is favoured in terms of flowtechnology. The outer flow director-free region may, for example, beincreased in that the number of flow directors arranged on theperipheral side is increased.

To adjust defined flow conditions in the primary tube gap, it may bepreferred if the at least one flow director is connected downstream inthe flow direction of the primary air flow from a swirling device toimpress a swirl on the primary air flow. The swirling device is, in thiscase, in particular provided in the primary tube gap even if the flowcan basically already be made to rotate by being supplied to the primaryair gap. The impressing of the swirl on the primary air flow may takeplace by means of swirling bodies, for example in the form of guidevanes or guide plates. These may preferably be inclined by 20° to 30°,in particular about 25°, compared to the longitudinal direction of theprimary tube.

The at least one flow director is preferably provided in front of adeflection device in the flow direction, so the primary air flow of thedeflection device can be supplied in a suitable manner. In this case,the flow director may, if necessary, be provided directly in front ofthe deflection device. It may even be provided that the flow directorand the deflection device are connected to one another in order to ruleout a possible impairment of the flow in the intermediate space. Inorder to avoid increased abrasion by fuel particles, wear-resistantmaterials, such as hard welding applications or ceramics, may be usedfor the flow directors.

A flame-holder projecting inwardly into the flow of the primary tube maybe provided at the outlet-side end of the primary tube to stabilise theflame. The edge of the flame-holder preferably pointing radiallyinwardly may be continuous or interrupted. A toothed edge, which canproduce a high degree of turbulence, is also conceivable.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail below with the aid ofdrawings showing only embodiments. In the drawings

FIG. 1 shows a first embodiment of the burner according to the inventionin a longitudinal section;

FIG. 2 shows a detail of the burner according to FIG. 1 in alongitudinal section;

FIG. 3 shows a detail of a second embodiment of the burner according tothe invention in a longitudinal section;

FIG. 4 shows a detail of a third embodiment of the burner according tothe invention in a longitudinal section; and

FIG. 5 shows a detail of a fourth embodiment of the burner according tothe invention in a longitudinal section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a longitudinal section through a burner 1, which isarranged in a wall W of a combustion chamber F. The inner part of theburner 1 from FIG. 1 is shown to an enlarged scale in FIG. 2 forimproved clarity.

A core tube 2, in which a burner gun, not shown, can be provided, isprovided in the centre of the burner 1. Other devices are also possible,which are shown here purely schematically. The core tube 2 is arrangedconcentrically with respect to a primary tube 3, so a peripheralconcentric primary tube gap 4 is provided between the core tube 2 andthe primary tube 3. A mixture of particulate biomass and combustionmeans, the primary air, is supplied to said primary tube gap by devices,not shown. Provided in the primary tube gap 4 is a swirling device 5 inthe form of guide vanes which are set at about 25° relative to thelongitudinal extent of the primary tube and which make the primary airflow rotate. The biomass particles then migrate in the flow direction,because of centrifugal forces, to the outside, where the biomassparticle concentration increases, while it accordingly decreases in aregion close to the core tube. A flame-holder 7, which defines theoutlet opening 8 of the primary tube 3, is provided at the outlet-sideend 6 of the primary tube 3. A toothed edge 9, which points radiallyinwardly, is provided on the inside of the flame-holder 7 and comes intocontact with the primary air flow and the biomass particles and,following this, ensures a swirling of the flow, which is indicated inFIG. 1 by the sharply curved arrows A.

A secondary tube 10 which, with the primary tube 3, forms a secondarytube gap 11, is provided concentrically with respect to the primary tube3. The secondary tube gap 11 has secondary air flowing through it, saidsecondary air having a swirl impressed on it by means of swirlingdevices 12 in the form of guide vanes set relative to the longitudinalextent of the primary tube in the secondary tube gap 11. The secondaryair does not have to be air in the actual sense. Provided at theoutlet-side end 13 of the secondary tube 10 is a secondary groove 14,which is a conical widening of the secondary tube 10 and deflects thesecondary air flow radially outwardly.

Provided on the outlet-side end 6 of the primary tube 3 is an outwardlypointing primary groove 15 in the form of a conical widening, whichcontributes to the outward deflection of the secondary air flow andleads to a stalling at the flame-holder 7. This stalling assists theconfiguration of the turbulent swirling of the biomass particles afterthe flame-holder 7, as is shown by the arrows B in FIG. 1.

Arranged concentrically with respect to the secondary tube 10 is atertiary tube 16, which, with the secondary tube 10, forms a tertiarytube gap 17. The tertiary air is guided to the combustion chamber F inthe tertiary tube gap 17, this not having to be air in the traditionalsense, which is made to rotate by means of swirling devices 18 in thetertiary tube gap 17. The tertiary tube 16, at its outlet-side end 19,has a conical widening, which is also called a muffle 20 and preferablyhas a larger angle of inclination than the secondary groove 14. Themuffle 20 is used to deflect the tertiary tube flow outwardly. For thepurpose of cooling, cooling lines L associated with the muffle 20 areprovided in the wall W of the combustion chamber F. In the shown and tothis extent preferred burner 1, the secondary groove 14 is set backinwardly relative to the muffle 20. The secondary groove 14 could,however, also be configured aligned with the muffle 20, in particularflush with the wall W of the combustion chamber F.

The outlet-side end 21 of the core tube 2 does not only endsignificantly in front of the flame-holder 7. The core tube 2, at theoutlet-side end 21, also has a conical taper 22. The axial spacing Dbetween the core tube 2 and the flame-holder 7, in the shown and to thisextent preferred burner 1, is at least equal to, if not greater than,the radial spacing R between the core tube 2 and the primary tube 3, inother words the width of the primary tube gap 4.

Accordingly, the outer diameter of the core tube 2 in the region of theoutlet-side end 21 decreases with an increasing closeness to theoutlet-side end 21 in the longitudinal direction. In the shown and tothis extent preferred embodiment, the conical taper 22 at theoutlet-side end 21 has a constant angle of inclination a ofsubstantially 7°. As a result of this configuration of the core tube 2and the axial spacing D between the core tube 2 and the flame-holder 7,a part flow of the primary air close to the core tube is deflected atthe outlet-side end 21 of the core tube 2 and thereafter in thedirection of the axial core region of the burner 1. A centring of theprimary air flow at the outlet-side end of the core tube 2, inparticular, however, at the outlet-side end of the primary tube 3, thustakes place. This centring, as illustrated by the arrows C in FIG. 1,leads to a part of the primary air being deflected centrally around theflame-holder 7, in particular around the edge 9, which is directedinwardly, of the flame-holder 7, without this part flow arrivingdirectly in the highly turbulent particle-rich flow region produced bythe flame-holder 7. At a later time, at which the centrally deflectedpart flow of the primary air is located further in the interior of thecombustion chamber F, the deflected part flow may, however, very wellcome into close contact with the fuel particles, if necessary, in orderto oxidise them.

FIG. 3 shows a detail of a burner 30 in a longitudinal section inaccordance with FIGS. 1 and 2. The same components have been given thesame reference numerals here. The important difference between theburners 1, 30 shown in FIG. 1 and FIG. 3 is that the core tube 2,peripherally on its outer lateral surface 31, has a plurality of flowdirectors 32, which are thin in the peripheral direction. The flowdirectors 32 extend parallel to the longitudinal extent of the burner 30or the core tube 2 and therefore deflect a part of the primary air inthe axial direction.

The flow directors could, however, alternatively also be inclined to theleft or right, i.e. extend both in the longitudinal direction andtransverse to the longitudinal direction of the primary tube 2,similarly to that which is the case with the swirling devices. Dependingon in which direction and with which incline the flow directors areinclined in the peripheral direction of the core tube, the swirl of thepart of the primary air flow close to the core tube is intensified orweakened. An incline of greater than 45° to 90° is basically lesspreferred here as the primary air flow is thus clearly decelerated.

The flow directors 32 of the shown and to this extent preferred burner30 allow the rotation of the primary air to be eliminated at least for apart of the primary air flow close to the core tube. In the burner 30shown, the outer primary air part flow adjoining the primary tube 3 isnot influenced by the flow directors 32. This primary air flow thuscontinues to rotate. For this purpose, the radial extent of the flowdirectors 32 in the shown and to this extent preferred burner 30 merelycorresponds to about 40% of the radial spacing R between the core tube 2and the primary tube 3.

The substantially axial core flow in the primary tube gap 4 isparticularly well deflected into a central region of the burner 1 by theconical region 22 of the core tube 2 and the axial spacing D from theflame-holder 7, as indicated by the arrow C in FIG. 3.

FIG. 4 shows a detail of a burner 40 in the longitudinal section, whichin addition to the burner 30 according to FIG. 3, has a deflectiondevice 41. The deflection device 41 is associated with the outlet-sideend 21 of the core tube 2 and forms a concentric annular gap adjoiningthe core tube 2. In the shown and to this extent preferred burner 40,the deflection device 41 covers the conically tapering portion 22 of thecore tube 2, which is formed in the embodiment by a reduction in thematerial thickness of the core tube 2. Before the conically taperingportion 22, the deflection device 41 forms an inlet region 42, in whichthe flow is oriented substantially axially, but not radially. The inletregion 42 may be formed by a concentric tube sleeve. In the region ofthe conically tapering portion 22 of the core tube 2, the deflectiondevice 41 in the shown and to this extent preferred burner has a portiontapering at the same angle of inclination a as the core tube 2. So thatthe flow cross section in the deflection device 41 does not decrease toosharply, the conical portion of the deflection device 41 may also beslightly less inclined, if necessary, than the conical portion 22 of thecore tube 2, so a constant flow cross section is provided, for example,in the deflection device 41. The deflection device 41 is preferablyconfigured as an axially peripheral component, which ends in the sameplane as the core tube 2. The deflection device 41 is spaced apart fromthe flow directors 43 and, in the shown and to this extent preferredburner 40, has a substantially similar radial overall height as the flowdirectors 43.

FIG. 5 shows the detail of a burner 50, in which the flow directors 51arranged distributed over the periphery of the core tube 2 are directlyconnected to the deflection device 52. Put more simply, the flowdirectors 51 guide the part flow close to the core tube in the primarytube gap 4 into the deflection device 52, which is configured as anaxially peripheral component. In the burner 50 shown in FIG. 4, thedeflection device 52 extends further in the direction of the outlet-sideend 6 of the primary tube 3 or the flame-holder 7, than the core tube 2.The deflection device 52 thus ultimately projects relative to the coretube 2 in the flow direction for sealing off relative to the turbulencesproduced by the flame-holder 7.

1. A burner for particulate fuel, in particular made of biomass,comprising a primary tube and a core tube arranged in the primary tube,wherein the primary tube and the core tube form a primary tube gap,wherein the primary tube gap is configured to guide a flow ofparticulate fuel and gaseous combustion means from an inlet-side end toan outlet-side opening of the primary tube, and wherein at least onedevice is provided for centring the flow within the primary tube in theregion of the outlet-side end of the primary tube.
 2. The burneraccording to claim 1, wherein the core tube, viewed in the longitudinaldirection of the burner, ends before the primary tube and wherein theaxial spacing between the outlet-side ends of the core tube and primarytube in the longitudinal direction of the primary tube is at least 50%,preferably at least 75%, in particular at least 100%, of the mean widthof the primary tube gap.
 3. The burner according to claim 1, wherein thecore tube is configured tapering toward its outlet-side end.
 4. Theburner according to claim 3, wherein the core tube tapers continuouslytoward its outlet-side end.
 5. The burner according to claim 4, whereinthe core tube tapers conically toward its outlet-side end, preferably atan angle of inclination α of less than 20°, in particular less than 10°,if necessary approximately 7°.
 6. The burner according to claim 1,wherein a deflection device is provided at the outlet-side end of thecore tube in the primary tube gap to deflect the part of the flow guidedin the primary tube gap close to the core tube inwardly.
 7. The burneraccording to claim 6, wherein the deflection device is configured todeflect about 30% by volume to 70% by volume, preferably 40% by volumeto 60% by volume, of the flow guided in the primary tube gap.
 8. Theburner according to claim 6, wherein a flow channel is provided betweenthe core tube and the deflection device and wherein the free flow crosssection of the flow channel remains substantially constant in the flowdirection.
 9. The burner according to claim 1, wherein at least one flowdirector is provided in the primary tube gap to influence the swirl of apart of the flow guided in the primary tube gap close to the core tube.10. The burner according to claim 9, wherein the at least one flowdirector is oriented in the longitudinal direction of the primary tubeand, preferably, is configured to be much wider in the longitudinaldirection than in the peripheral direction.
 11. The burner according toclaim 9, wherein the at least one flow director is inclined transverseto the longitudinal direction of the primary tube.
 12. The burneraccording to claim 9, wherein the at least one flow director is providedwithin the inner 80%, preferably 70%, in particular 60%, of the width ofthe primary tube gap.
 13. The burner according to claim 9, wherein theat least one flow director is provided downstream in the flow directionof a swirling device preferably arranged in the primary tube gap, andwherein the swirling device is provided to impress a swirl on the flowguided in the primary tube gap.
 14. The burner according to claim 7,wherein the at least one flow director is provided in the flowdirection, preferably directly in front of the deflection device. 15.The burner according to claim 7, wherein a flame-holder projectinginwardly into the flow of the primary tube is provided on theoutlet-side end of the primary tube.